These waste waters contain substantial amounts of dissolved organic compounds, CO.sub.2, H.sub.2 S and NH.sub.3 which must be removed in several steps before the waste waters can be discharged to conventional treatment devices or to rivers or other receiving streams.
The dissolved organic compounds are phenols and other oxygen-containing compounds such as ketones and aldehydes, and also aliphatic carboxylic acid (fatty acids), which are particularly common in waste waters from the lignite gasification or degassing process. Other dissolved organic compounds are nitrogen-containing compounds, such as pyridine decomposition products, and also sulfur compounds. The dissolved inorganic compounds are mainly gases e.g. CO.sub.2, H.sub.2 S and NH.sub.3.
It is known that the dissolved organic compounds in such waste waters can be extracted with water-immiscible or substantially water-immiscible solvents. Suitable solvents are e.g. benzene, tricresyl phosphate, or diisopropyl ether. When a waste water has been extracted with a solvent, the residual impurities of the waste water are small amounts of solvent and the volatile inorganic compounds mentioned above. These residual impurities can be removed by contacting the water with steam.
Waste water which has been freed from ammonia can be treated in a conventional waste water treatment device characterized by a biological or a mechanical treatment step or discharged in a river course or other disposal system.
Various processes for the treatment of waste waters have been used for many years and have proved to be quite reliable and adaptable to varying conditions and the properties of the treated waste waters. However, waste waters of the processing of lignite may contain substantial amounts of fatty acids. These compounds cannot be satisfactorily extracted during the extraction of organic compounds with a solvent and remain in phenol-free waste waters and bind the ammonia present in such waste waters. Lime milk, sodium carbonate or the like can be added to the neutralized waste water to make it alkaline to such an extent that the ammonia present in the waste water is freed and can be removed.
These known processes meet environmental requirements but are too expensive and complicated, especially as far as the the ammonia removal is concerned. The reason for this is that ammonia is removed in a single step in the known gas/liquid contacting systems and therefore can be found in the head product of a contacting column only in small concentrations. Hence, the use of sulfuric acid has been necessary to bind the ammonia and precipitate it as ammonium sulfate.
Efforts have been made to obtain higher concentrations of ammonia in the head product of a contacting column by the use of refluxing in an extra column portion. However, it has been found repeatedly that then a zone of higher concentration of CO.sub.2 is also established, which can lead to a flooding in this zone of the column. The high concentration of CO.sub.2 in this zone can also lead to a high concentration of CO.sub.2 in the head product which, in turn, can lead to crystal deposits of ammonium carbonate in the condenser and in its inlet and outlet portions, especially in the coldest areas thereof. Such deposits can ultimately result in obstruction of the passageways.